Many papers following forming and drying are subjected to functional coating. The present invention is applicable to all such coating operations, including the forming of barrier or pigment coats and top coats.
Most coatings are applied to the base paper in the form of water suspensions, although organic solvents or carriers can also be employed. After application, the coating is dried and then usually is calendered to smooth the surface, control surface texture, and develop a glossy finish. In the calendering operation, the paper is passed successively through the nips of a stack of as many as 12 or more hard steel or alternating hard steel and soft rolls.
Typical adhesives or binding agents for pigment and other cured coatings include casein, soy or alpha protein, starch, various modified rubber latices and other emulsions such as styrene-butadiene latex, emulsions based on acrylics and polyvinyl acetate.
Historically, these coatings have often been cured with aldehydes and aldehyde amine resins. Depending upon the heat and formulas, varying and often low degrees of cure are initially obtained. Several methods are available to speed up cure, such as using a catalyst. This, however, has the effect of reducing pot life, causing many production problems. It is possible to use higher levels of curing agents, but this often adversely affects other physical properties of the coating or paper product. Another method employed is the use of heat to promote a cure.
The use of acids to catalyze the action of the curing agent is well known. In the paper industry, such acid catalysts are often employed by installing water boxes on the calender stacks to add a small amount of the dilute acid to the already coated paper. This causes a full cure very quickly, without which the treatment could take weeks.
However, not all coating machines have such water boxes. It is an object of the present invention to provide a means for obtaining a rapid and controlled cure of applied coatings in a novel and facile manner.
Prior U.S. Pat. No. 3,632,296, assigned to Cluett, Peabody & Co., Inc., relates to a method for imparting durable press characteristics to textile fabrics containing some reactive fibers (e.g., cellulosic) in which resins or other cross-linking agents are applied to the fabric, optionally in the presence of a catalyst, and the fabric is heated to a temperature at which cross-linking of the reactive fibers occurs. A delayed cure is obtained by encapsulating the cross-linking material and/or catalyst in microcapsules. On the application of heat or pressure to the sensitized fabric, the encapsulated material is released to obtain curing. The method of this patent is limited to the treatment of fabrics. Also, controlling the rate of cure is not an aspect of this patent.